Projection engineering (Sept 1929-Nov 1930)

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Page 16 Projection Engineering, October, 1929 Fig. 4. The course of the film through the machine may be seen from this photo which shows also the concentrated light source and the lens assembly. projector head. A separate gear coupling is provided at the upper end for each type of head used. The other vertical shaft is connected by a selfaligning shaft to a set of gears that drives the sound sprocket which pulls the film through the sound reproducing unit. The horizontal shaft is connected through a chain and sprocket drive to the film take-up mechanism. This consists of a shaft, on which the film reel is mounted driven through a frictiondisc type of clutch. A hold-back sprocket mechanism, which serves to prevent any uneven pull of the film take-up mechanism from affecting the uniform velocity of the film as it passes the lens system, is also connected to this drive. It is mounted in the lower film reel housing, as may be seen in Fig. 4. Mounted on top of the film magazine casting is a pivoted bracket carrying the sound-film reproducing unit, to which is fastened the projector head. The same bracket also mounts the projector arc lamp. It is pivoted about the center of the sound sprocket and may be tilted from the horizontal to 40° below. A jack screw is provided, and adjustment at any angle is obtained by means of a hand wheel. The sound-film reproducing unit consists of a sprocket for pulling the film at a uniform velocity past an aperture plate, an exciting lamp, a lens system, and a photoelectric cell. The general arrangement may be seen from the ^r 4LITSSSg:»l!SiWI0ExA'uONG diagram of Fig. 5 and from the views of Figs. 4 and 6. Light from the concentrated filament source is focused by a condensing lens on a narrow slit as shown in Fig. 7. An image of this slit is then focused on the film by a high quality objective lens, and light passing through the sound track on the film falls on the photoelectric cell. When polarized by a proper voltage, the current output of the photoelectric cell is proportional to the light falling upon it. The cell circuit is inherently of high impedance and small energy output so that it is very necessary to guard against local electrical interference. To accomplish this a vacuum tube amplifier is mounted on the machine as closely as possible to the cell. The amplifier serves the double purpose of increasing the energy level and making the energy available across a low impedance circuit. The vacuum tubes are mounted on a spring-suspended platform to prevent vibration from causing noise in the output circuit. Fig. 7. The optical system is simple in detail but very accurate in adjustment. Fig. 6. To avoid local electrical disturbances the amplifier is mounted directlv beneath the photoelectric cell. Their relative position is here plainly seen. Due to the addition of several flywheels and various mechanisms to the regular projector mechanism, a longer time than unual is required for the machine to come to rest after the power is shut off. This is objectionable at times of film breakage, and to correct it a brake has been provided controlled by a foot pedal shown in Figs. 1 and 6. Depression of this pedal first shuts off the power and then applies a brake to the motor flywheel. The machine is restarted by lifting the brake pedal, which first releases the brake and then closes the motor switch. Many special features have been provided in addition, such as adjustable legs, allowing a variation of ten inches in height, and a mechanism for shifting the projector a small amount in the horizontal plane to compensate for PHOTOELECTRIC CELl Fig. 5. Details of the sound-film reproducing unit are here shown. the difference in width of the picture on the silent and sound-picture films. Advantage has been taken of all recent improvements and experience so that the new Western Electric reproducing machine embodies the latest improvements in sound-picture projectors. RADIO EYE HARDER TO FOOL THAN THE RADIO EAR TELEVISION is something quite different from the usual sound broadcasting. Both arts may be based on the same broad radio essentials, but in practice many differences are encountered. For one thing, it is far easier to fool the radio ear, with just an approximation of music and speech, than it is to fool the radio eye or television broadcasting, with incorrect pictorial values. The engineers of the Jenkins Television Corporation in the Jersey City laboratories have found the practical problems of television far more difficult than were at first anticipated. In the pick-up of images, for instance, the usual sound broadcasting practice was followed, working with the equipment in the open laboratory. However, the many inductive interferences picked up by the delicate equipment and exposed conductors soon convinced the engineers that they had to work in grounded copper mesh cages and with thoroughly shielded apparatus and conductors. Again, the finest resistancecoupled amplifiers employed for socalled distortionless amplification in sound and laboratory practice, were found inadequate for the extraordinary requirements of television. Special amplifiers had to be developed. Still again, the first film pickup machines, utilizing simple scanning discs, proved to have an angular movement in scanning the film frames, due to the arc traveled by the scanning holes, together with the steady downward movement of the film image itself. Therefore, new and improved scanning methods had to be developed.